In addition to having to know where Oswald was at the time of the assassination, the conspirators who allegedly framed him for shooting Tippit also had to know he had no alibi.
If they framed him for shooting Tippit and he has an alibi - he was back with Marina with the Paine's, he' still on the bus stuck in traffic on Houston Street, he's watching the news with Roberts at his rooming house, he's at the movies - then he can't be framed. He has an alibi.
So how did they know not only that he could be framed for shooting JFK but also for shooting Tippit?
They couldn't. Here's what the evidence shows: Oswald shot JFK. And then he fled from the building. He got his revolver with extra bullets. Then he shot Tippit.
Maybe he had help, we'll never know. But he shot JFK and Tippit because, as he stated numerous times, he hated the US. And he struck back at the symbol - JFK - that represented the system he hated.